Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting
Premium Income	$ 2,800.7	$ 2,688.0	$ 8,412.5	$ 8,138.9
Operating Segments
Segment Reporting
Premium Income	2,800.7	2,688.0	8,412.5	8,138.9
Operating Segments | Unum US
Segment Reporting
Premium Income	1,846.6	1,755.4	5,545.3	5,334.9
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting
Premium Income	489.9	499.2	1,484.0	1,511.5
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting
Premium Income	326.7	286.0	969.6	853.6
Operating Segments | Unum US | Group Life
Segment Reporting
Premium Income	498.1	468.3	1,493.2	1,404.5
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting
Premium Income	53.3	48.9	160.1	146.3
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting
Premium Income	250.9	231.6	756.4	700.2
Operating Segments | Unum US | Individual Disability
Segment Reporting
Premium Income	144.6	140.0	433.9	475.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting
Premium Income	83.1	81.4	248.1	243.4
Operating Segments | Unum International
Segment Reporting
Premium Income	291.6	281.1	867.6	798.9
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting
Premium Income	107.6	111.3	319.9	319.4
Operating Segments | Unum International | Group Life
Segment Reporting
Premium Income	78.9	71.9	233.1	201.6
Operating Segments | Unum International | Supplemental
Segment Reporting
Premium Income	51.3	47.4	152.4	136.3
Operating Segments | Unum International | Unum Poland
Segment Reporting
Premium Income	53.8	50.5	162.2	141.6
Operating Segments | Colonial Life
Segment Reporting
Premium Income	470.0	456.5	1,420.1	1,375.9
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting
Premium Income	251.6	247.5	758.9	743.7
Operating Segments | Colonial Life | Life
Segment Reporting
Premium Income	127.2	118.7	386.6	360.7
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting
Premium Income	91.2	90.3	274.6	271.5
Operating Segments | Closed Block
Segment Reporting
Premium Income	192.5	195.0	579.5	629.2
Operating Segments | Closed Block | Long-term Care
Segment Reporting
Premium Income	161.4	158.6	480.2	510.7
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting
Premium Income	$ 31.1	$ 36.4	$ 99.3	$ 118.5